(a) Field of the Invention
The present invention relates to a residual frequency error, phase error, timing error, and signal amplitude variation tracking apparatus and method. More particularly, the present invention relates to a residual frequency error, phase error, timing error, and signal amplitude variation tracking apparatus and method for enhancing a phase and time offset tracking performance and a tracking speed in orthogonal frequency division multiplexing (OFDM) systems.
(b) Description of the Related Art
In a representative IEEE 802.11a standard of a conventional OFDM packet communication, an initial preamble is used to synchronize frequency and time. However, it is necessary to provide various synchronization tracking algorithms so as to overcome a lowering of system performance which is caused by a residual frequency error, a sampling frequency error, a frequency shift, or the like, due to an estimating error of an initial frequency error and a signal amplitude variation in one frame.
First, in such an OFDM scheme, the effect of carrier frequency error will be explained as follows.
In the case of there being carrier frequency error, the carrier frequency error causes a predetermined phase shift for all subcarriers and an inter carrier interference (ICI) therebetween. When a transmit signal Xk passes through a channel having a channel frequency response Hk and has a frequency error ε, a receive signal (Yk) of the k-th subcarrier is given as Equation 1.
                              Y          k                =                                            X              k                        ⁢                          H              k                        ⁢                                          sin                ⁢                                                                  ⁢                πɛ                                            N                ⁢                                                                  ⁢                sin                ⁢                                                                  ⁢                                  (                                      πɛ                    /                    N                                    )                                                      ⁢                          ⅇ                              jπɛ                ⁢                                                                  ⁢                                                      (                                          N                      -                      1                                        )                                    /                  N                                                              +                      I            k                    +                      W            k                                              (                  Equation          ⁢                                          ⁢          1                )            
In Equation 1, N indicates a point number of a fast Fourier transform (FFT), that is, the total number of subcarriers. The Ik is an ICI and is given as Equation 2.
                              I          k                =                              ∑                                          l                =                0                            ,                              1                ≠                k                                                    N              -              1                                ⁢                                          ⁢                                    X              l                        ⁢                          H              l                        ⁢                                          sin                ⁢                                                                  ⁢                πɛ                                            N                ⁢                                                                  ⁢                sin                ⁢                                                                  ⁢                                  (                                      π                    ⁢                                                                                  ⁢                                                                  (                                                  l                          -                          k                          +                          ɛ                                                )                                            /                      N                                                        )                                                      ⁢                          ⅇ                              jπɛ                ⁢                                                                  ⁢                                                      (                                          N                      -                      1                                        )                                    /                  N                                                      ⁢                          ⅇ                              jπ                ⁢                                                                  ⁢                                                      (                                          l                      -                      k                                        )                                    /                  N                                                                                        (                  Equation          ⁢                                          ⁢          2                )            
In the case of there being a time offset, the time offset causes a phase error to be increased in proportion to an index of the OFDM subcarrier and the time offset is in proportion to a slope of the phase error to the index of the subcarrier.
Accordingly, when one FFT section has an error of I=0 and I≠k, the relation of frequency domain signals (Yk,τ) having an error and a frequency domain signal (Yk) having no error is given as Equation 3.Yk,τ=Ykej2πτk/N  (Equation 3)
In Equation 3, it is given as
      k    =          -              N        2              ,  …  ⁢          ,  0  ,  …  ⁢          ,            N      2        -    1.  Accordingly, although the time offset is the same, the time offset less affects low-frequency elements which are in the vicinity of DC and more affects high-frequency elements.
When such errors are not tracked and compensated, such errors less affect the beginning part of the frame. However, since such errors are accumulated as the frame is processed toward the ending part, lowering of performance is severe at the ending part of the frame.
Therefore, a pilot symbol having a fixed pattern is used to estimate such errors.
The pilot symbol has been used to estimate a phase in a conventional feedback scheme where the mean value and the slope of the estimated phase are passed through a loop filter and then the pilot symbol is corrected. The feedback scheme can obtain a desired system performance assuming the residual carrier error is very small and the time offset variation is very small.
U.S. Pat. No. 6,122,246 (registered on Jul. 25, 1999), entitled “Apparatus and Method for Clock Synchronization in a Multi-Point OFDM/DMT Digital Communications Systems” is incorporated herein by reference.
This patent discloses that in a communication system using an OFDM scheme, a receive port estimates a frequency error based on a pilot signal, detects a subsymbol including the pilot signal, obtains a phase difference between the detected pilot signal and an adjacent pilot signal and obtains a sampling frequency error based on the above information, in order that a transmit port estimates and tracks a difference between a clock frequency of the receive port and a frequency of the receive port using signal information on a subchannel in which a pilot exists and an adjacent channel.
“Carrier Frequency Acquisition and Tracking for OFDM Systems,” IEEE Transactions on Communications, vol. 44, no. 11, pp. 1590-1598, November 1996 (M. Luise etc.) discloses a method for tracking a frequency error by detecting a phase difference between a demodulation symbol of subcarriers and a received symbol.
Also, “Timing recovery for OFDM transmission,” IEEE Journal on Selected Areas in Communications, vol. 18, no. 11, pp. 2278-2291, November 2000 (Baoguo Yang) discloses a method for tracking a symbol timing variation due to a clock frequency error using a delay-locked loop (DLL) to synchronize a frequency of a sampling clock.
As described above, the OFDM packet communication system, such as a wireless LAN, acquires a synchronization using a channel estimating value and a frequency error estimating value obtained from a preamble which is placed at the beginning part of one frame. However, when the frame is long, errors are accumulated causing the system performance to be severely lowered. In addition, the time offset, which is regularly increased or decreased according to the time and is caused by the fixed frequency error and the sampling frequency error, must be estimated and then corrected using a pilot symbol of a frequency domain. In Equation 1 and Equation 2, a small amount of the frequency error appears as a predetermined phase error and the time offset appears as a phase error having a slope.
The prior art can be applied when the error is small enough to be corrected by the feedback scheme without lowering the system performance. However, in a real environment, a frequency shift and a phase shift may abruptly occur due to a radio frequency (RF) circuit and an analogue circuit as well as such error, and also, a signal amplitude variation may occur due to a phenomenon where the signal amplitude is gradually increased when a transceiver amplifier turns on. Accordingly, the prior art still has a problem of lowered performance of the OFDM system.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.